This invention relates to a device for detecting the speed of an electric motor such as a stepping motor.
While ordinary electric motors make at least one revolution, some of the special motors are turned, for instance, through 150.degree. smaller than 360.degree. (one revolution) at a time. Thus, the speed of a special motor of this type is detected in correspondence to the rotation range of 150.degree.. A tachometer generator commercially available may be used to detect the speed of the special motor as it is; however, it is disadvantageous in that it is expensive and a device using a special motor unavoidably become expensive and bulky.
A variety of motor speed detecting devices of generator type have been proposed for special motors. However, the conventional devices suffer from drawbacks that the manufacturing process is intricate and therefore the devices are expensive, and that the range in which a speed detection signal is produced is small, and the speed detection signal is not constant; i.e., the devices are low in performance.